1. Field of the Invention
This invention relates to an electric power steering unit to be used for a vehicle and, more particularly, it relates to a field of technology effectively applicable to electric power steering gears to be used for rack and pinion type steering systems.
2. Related Art Statement
In recent years, most vehicles are equipped with a so-called power steering gear, and various type power steering unit, such as a hydraulically or electrically operated type power steering unit has been designed to assist the steering power of the vehicle. In these electric power steering units, as one for applicable to rack and pinion type steering systems, a unit which the steering assisting power is obtained by an electric motor arranged coaxially with a rack-shaft, as Japanese Patent Application Laid-Open No. 8-98451, is known.
Such an electric power steering unit shown in FIG. 5, comprises an electric motor 52 arranged coaxially with a rack-shaft 51 so that the steering assisting power generated by the electric motor 52 is transmitted to the rack-shaft 51 by way of a ball screw mechanism 53. Then, the guiding wheels of the vehicle can be turned by utilizing both the manual steering power of the driver and the steering assisting power.
The rack-shaft 51 is linked to the guiding wheels (which hereinafter may simply be referred to as "the wheels") typically by way of tie rods or knuckle arms arranged at the respective opposite ends thereof and also linked to the steering column 54 that is coupled to the steering wheel (which hereinafter may be referred to as "the handle") by way of a rack and pinion gear so that it may be reciprocatively moved in the horizontal directions of FIG. 5 as the steering operation by the driver. The electric motor 52 has a cylindrical yoke 55 containing coaxially therein a cylindrical armature shaft 56 and a field device 57 and is fed with power from a power supply section 58. The field device 57 comprises magnets 59 arranged on the inner peripheral portion of the yoke 55 and an armature core 60 arranged on the outer peripheral portion of the armature shaft 56. The rotary power generated by the electric motor 52 is transmitted to the rack-shaft 51 by way of a ball screw mechanism 53 arranged at the left end of the armature shaft 56 in FIG. 5. Note that the armature shaft 56 is supported at a right side portion thereof by an angular bearing 65 held within housing 61.
The ball screw mechanism 53 has a well known constitution, which comprises a large number of balls 64 arranged between a nut section 62 and a screw section 63 thereof, the nut section 62 being press-fit into and caulked against the armature shaft 56. With this arrangement, the rotary power of the electric motor 53 is transmitted to the rack-shaft 51 by way of the nut section 62, the balls 64 and the screw section 63 to produce axial reciprocative power there, which is then used to assist the steering power.
A power steering unit having the above described configuration is then mounted onto the vehicle by securely fitting the housing 61 on the right side of FIG. 5 to the vehicle main body and fitting the yoke 55 on the left side of FIG. 5 by a bracket 70 as in the case of a hydraulic power steering unit. More specifically, the housing 61 is directly fitted to the vehicle by securing a fitting section 69 that is an integral part of the housing 61 to the vehicle main body by means of screws. On the other hand, the yoke 55 is fitted to the vehicle by holding the end of a pipe-shaped diameterically small portion 55a (on the left side in FIG. 5) of the yoke 55 under the bracket 70 having a U-shaped cross section at a middle portion thereof. Note that an elastic member (not shown) typically made of rubber is arranged around the diametrically small portion 55a of the yoke 55 so that the yoke 55 is held under the U-shaped area of the bracket 70 with the elastic member disposed therebetween. The strength with which the unit is secured largely depends on the strength of securing the housing 61 because the housing 61 is by far securer than the bracket 70 when fitted to the vehicle main body.
A hydraulic power steering unit typically comprises a combination of pipe-shaped member and a housing, the housing being located close to the steering column. Since, unlike the housing, it is not possible to provide the pipe-shaped member with a fitting section, the member has to be secured in position by means of a bracket. If, on the other hand, the housing is secured by means of a bracket, the pipe-shaped member is required to show an enhanced strength when fitted in position and a further housing has to be located at the side of the pipe-shaped member for the sake of the strength. Thus, with a hydraulic power steering unit, it is the most common practice to rigidly secure the housing to the vehicle main body and hold the pipe-shaped member in position by means of a bracket because this technique is advantageous particularly in terms of cost and this practice is normally followed by electric power steering units.
As described above, with a conventional power steering unit, a component thereof located close to the steering column 54 of the motor vehicle, that is, the housing 61 is rigidly and directly secured to the vehicle main body. However, while such a constitution shows a greater holding strength than when it is secured by means of a bracket, it is then less apt to damp the sounds and vibrations transmitted to it from the vehicle main body. Therefore, a power steering unit shown in FIG. 5 is accompanied by a problem that it cannot damp the controlling sounds and vibrations of the electric motor 52 and hence they may be easily transmitted to the steering column 54. It would be appreciated that, while the noises generated outside the motor vehicle are apt to be drowned by the sounds of the moving vehicle and its engine so that they may provide less nuisance to the passengers in the vehicle, the sounds and vibrations transmitted to the steering column 54 can then be conveyed to the inside of the vehicle by way of the steering wheel. Therefore, the sounds are confined in the room of the vehicle and they give nuisances to the driver and passengers in the vehicle and possibly interfere with the steering feeling of the driver.
Additionally, since a conventional housing 61 as shown in FIG. 5 has a fitting section 69 projecting in a direction that is three-dimensionally different from the projecting direction of the member that holds the steering column 54 so that a costly mold having a complicated cavity profile has to be used for manufacturing the housing to push up the manufacturing cost.
Still additionally, motor vehicles of different types have different positions for securing the power steering unit to the vehicle main body. Therefore, since the fitting section 69 of the housing 61 is fixed, different housings in which the position of the fitting section 69 is different from each other should be prepared for motor vehicles of different types corresponding to the securing position. This means that different types of motor vehicle require different complicated molds and many kinds of products is required for each types. Accordingly, the management of the products is very troublesome and it raises the manufacturing cost of the unit.
An object of the present invention is to provide an electric power steering unit that can effectively prevent the sounds and vibrations of its electric motor from being transmitted to the steering wheel.
Another object of the present invention is to provide an electric power steering unit which can simplify the cavity profile of the molds and allow the housing of the steering column side to be commonly used in motor vehicles of different types.
The above-described and other objects, and novel feature of the present invention will become apparent more fully from the description of the following specification in conjunction with the accompanying drawings.